CVE-2026-53965

The record

The MCP PHP SDK (Composer package mcp/sdk) is the official Model Context Protocol SDK for PHP. In versions 0.5.0 through 0.7.0, the HTTP client transport reads a Server-Sent Events response stream incrementally and appends each chunk to an in-memory buffer with no upper bound. The buffer is only flushed when an SSE event delimiter, a double newline, is found, so a remote MCP server that streams response bytes without ever sending the delimiter causes the buffer to grow without limit. A malicious, compromised, or man-in-the-middle-controlled server that the client connects to over the HTTP transport can exploit this to exhaust the client process's memory, triggering a fatal allocation error or OS out-of-memory kill and denying service to the MCP client. The issue affects any client using HttpTransport against an untrusted server endpoint and does not require authentication or user interaction beyond initiating the connection. This issue is fixed in version 0.7.1.

Affected packages

Related vulnerabilities

• CVE-2026-113795 pypdf is a free and open-source pure-python PDF library. CWE-400
• CVE-2026-83400 Vulnerability in the Oracle Java SE, Oracle GraalVM for JDK, Oracle GraalVM Enterprise… CWE-400
• CVE-2026-112400 Pillow is a Python imaging library. CWE-400
• CVE-2026-111766 markdown-it is a Markdown parser. CWE-400
• CVE-2026-111759 markdown-it is a Markdown parser. CWE-400
• CVE-2025-52290 A NULL pointer dereference in the avpriv_h264_has_num_reorder_frames function of Ffmpeg… CWE-400
• CVE-2026-111066 AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. CWE-400
• CVE-2026-59951 The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context… CWE-400